1. Field of the Invention
The present invention generally relates to the field of molecular beam epitaxy (MBE). More specifically, the present invention relates to MBE nitrogen sources.
2. Discussion of the Related Art
Molecular beam epitaxy (MBE) is a well-established method of forming thin film depositions on crystalline substrates. The MBE process is performed by heating a crystalline substrate in a vacuum so as to energize the substrate's lattice structure. Then, an atomic or molecular mass beam(s) is directed onto the substrate's surface. When the directed atoms or molecules arrive at the substrate's surface the directed atoms or molecules encounter the substrate's energized lattice structure. The energetic lattice structure then accepts oncoming atoms, possibly after those atoms are separated and removed from molecule. Over time, the oncoming atoms form a film that is characterized by high crystallinity and excellent surface uniformity. Because of its utility and the high quality films produced, MBE is widely used in semiconductor research and in fabricating semiconductor devices, particularly semiconductor devices that benefit from thin-film depositions of elemental materials.
FIG. 1 illustrates an MBE apparatus in operation. As shown, the MBE apparatus includes a chamber 106. During operation, that chamber 106 is ultra highly evacuated. An effusion cell 100 then injects a beam 104 of gaseous atoms or molecules 102 into the chamber 106. The beam 104 is directed at a predetermined angle toward a heated crystalline substrate 110 that is mounted in the chamber 106.
Effusion cells 100 come in a variety of configurations that are beneficially tailored for the specific material(s) being deposited. Effusion cells 100 include a crucible 111 that contains an effusion material 112, for example aluminum, gallium, indium, or other compounds. The crucible 111 and its contents are then heated to effuse the material from an orifice 114 into the chamber 106 and onto the substrate 110. The intensity of the beam 104 is controlled by the temperature of the crucible 111, which is set by a heater 115 (not shown in FIG. 1, but see FIG. 3), and by a shutter 108 positioned in the chamber 106 near the orifice 114. The ultra-high vacuum environment (UHV) used in MBE is typically about 1×10−3 Pa. This enables a relatively low substrate temperature, typically about 750° C.
High purity gases, such as hydrogen, and high vapor pressure materials, such as arsenic, phosphorous, selenium, and sulfur, may be effused by thermally cracking them into dimers or into various atomic species. With gaseous materials, a valve can be used instead of the shutter 108 to control the intensity of the beam 104.
While FIG. 1 illustrates an MBE apparatus in operation, in practice a plurality of effusion cells 100 are used. In such cases, beams 104 from the individual effusion cells can be actuated individually or in conjunction with other beams to produce desired thin films on the substrate 110. In any event the species from the effusion cells travel across the MBE chamber 106 to the substrate 110. There, epitaxial growth occurs according to the controlling deposition kinetics.
MBE has proven useful for producing a wide range of epitaxial films. Of particular interest to the present invention are thin films that contain nitrogen. For example, FIG. 2 illustrates a vertical cavity surface emitting laser (VCSEL) having a nitrogen-containing layer that is suitable for MBE fabrication. As shown, an n-doped gallium arsenide (GaAs) or InP substrate 12 has an n-type electrical contact 14. An n-doped lower mirror stack 16 (a DBR) is on the substrate 12, and an n-type graded-index lower spacer 18 is disposed over the lower mirror stack 16. An active region 20 having a number of nitrogen containing quantum wells, for example GaInAsNSb, is formed over the lower spacer 18. A p-type graded-index top spacer 22 is disposed over the active region 20, and a p-type top mirror stack 24 (another DBR) is disposed over the top spacer 22. Over the top mirror stack 24 is a p-type conduction layer 9, a p-type cap layer 8, and a p-type electrical contact 26.
Still referring to FIG. 2, the lower spacer 18 and the top spacer 22 separate the lower mirror stack 16 from the top mirror stack 24 such that an optical cavity is formed. As the optical cavity is resonant at specific wavelengths, the mirror separation is controlled to resonate at a predetermined wavelength (or at a multiple thereof). At least part of the top mirror stack 24 includes an insulating region 40 that provides for current confinement. The insulating region 40 is usually formed either by implanting protons into the top mirror stack 24, or by forming an oxide layer. The insulating region 40 defines a conductive annular central opening 42 that forms an electrically conductive path through the insulating region 40.
In operation, an external bias cause an electrical current 21 to flow from the p-type electrical contact 26 toward the n-type electrical contact 14. The insulating region 40 and the conductive central opening 42 confine the current 21 such that it flows through the conductive central opening 42 to the active region 20. Some of the electrons in the current 21 are converted into photons in the active region 20. Those photons bounce back and forth (resonate) between the lower mirror stack 16 and the top mirror stack 24. While the lower mirror stack 16 and the top mirror stack 24 are very good reflectors, some of the photons leak out as light 23 that travels along an optical path. Still referring to FIG. 2, the light 23 passes through the p-type conduction layer 9, through the p-type cap layer 8, through an aperture 30 in the p-type electrical contact 26, and out of the surface of the VCSEL 10.
While MBE has proven useful for producing nitrogen-containing layers, such as found in the VCSEL 10, the available nitrogen sources are less than optimal. In practice, two types of nitrogen sources are available: elemental nitrogen and ammonia. Elemental nitrogen is derived from nitrogen plasma, while ammonia is thermally cracked on a heated crystalline substrate (On-Surface Cracking) to produce elemental nitrogen. Because of limitations of available nitrogen sources, new MBE nitrogen sources would be beneficial.